Although it has long been known that polymers can be mixed with appropriately modified clay minerals and synthetic clays, the field of polymer/clay nanocomposites has recently attracted great interest. Two major findings pioneered the interest in these materials: First, the report of a nylon-6/montmorillonite (mmt) material from Toyota research (Kojima et al. J. Mater. Res. 1993, 8, 1179 and J. Polym. Sci. Part A: Polym. Chem. 1993, 31, 983), where it was shown that very moderate inorganic loadings resulted in concurrent and remarkable enhancements of thermal and mechanical properties. Second, Giannelis et al. found that it is possible to melt-mix polymers with clays without the use of organic solvents (Chem. Mater. 1993, 5, 1694). Since then, the high promise for industrial applications for these composites has motivated vigorous research. This research has revealed that concurrent dramatic enhancements of many properties of polymeric materials can be achieved by the nanodispersion of inorganic silicate layers. These improvements are generally applicable across a wide range of polymers in instances where the property enhancements originate from the nanocomposite structure. Property improvements that could not be realized by conventional fillers were also discovered in these nanoscale materials. Examples include increased tensile strength, flex modulus, impact toughness, general flame-retardant characteristics (Gilman et al. Chem. Mater. 2000, 12, 1866), and a dramatic improvement in barrier properties (Manias et al. Macromolecules 2001, 34, 337).
There are three general methods known in the prior art for the preparation of polymer/clay nanocomposites, including (i) in situ polymerization (Usuki et al. J. Mater. Res. 1993, 8, 1179 and Lan et al. J. Chem. Mater. 1994, 6, 2216), (ii) solution blending (Jeon et al. Polymer Bulletin 1998, 41, 107), and (iii) melt blending (Giannelis, E. Adv. Mater. 1996, 8, 29). All of the methods are aimed at achieving single layer dispersion of the layered silicate in the polymer matrix, because high surface area is directly associated with the enhanced properties in polymer/clay nanocomposites. For the in situ polymerization method, the initiator or catalyst is usually pre-fixed inside the clay interlayer via cationic exchange, then the layered silicate is swollen by monomer solution. The polymerization occurs in situ to form the polymer right between the interlayers with intercalated and/or exfoliated structures.
Solution blending involves the use of a polymer solution (or a prepolymer in the case of insoluble polymers such as polyimide). The layered silicates (modified with organic surfactants) can be easily dispersed in an appropriate solvent. The polymer should also be soluble in the same solvent. When the solvent is evaporated (or the mixture precipitated), the sheets try to reassemble, kinetically trapping the polymer between them to form a nanocomposite structure.
In the melt blending process, the layered silicate is mixed with the polymer matrix in the molten state. If the layer surfaces are sufficiently compatible with the chosen polymer, the polymer enters into the interlayer space and forms either an intercalated or an exfoliated nanocomposite. No solvent is required in this technique. It is obviously the most desirable industrial method.
In general, the pristine state clay (i.e. unmodified clay) having highly hydrophilic polar surfaces is only miscible with hydrophilic polymers such as poly(ethylene oxide) and poly(vinyl alcohol). (Manias et al. Chem. Mater. 2000, 12, 2943; Vaia et al. Adv. Mater. 1995, 7, 154). To render clay miscible with hydrophobic (non-polar) polymers, one must modify it by exchanging the alkali counterions with cationic-organic surfactants, such as alkylammoniums, to form organophilic clay (Giannelis et al. Adv. Polym. Sci. 1998, 138, 107). The organic surfactant not only changes the clay from hydrophilic to hydrophobic surfaces by cation-exchange of the cations (Li+, Na+, Ca2+, etc.) between clay interlayers with onium ions in organic surfactants, it also expands the clay tight interlayer structure by increasing (001) d-spacing between the layers. Based on theoretical modeling (without any experimental results), Balazs et al. suggested the potential benefit of increasing the length of the surfactant (such as an end-functionalized chain with two terminal groups) to possibly promote the dispersion of bare clay sheets within the polymer matrix (Balazs et al. Macromolecules 1998, 31, 8370 and J. Chem. Phys. 2000, 113, 2479).
Experimentally, it is very common in polymer/clay nanocomposites to have a mixed nano-morphology, with both intercalated and exfoliated structures existing in the system. Intercalated structures are self-assembled, well-ordered multilayered structures where the extended polymer chains are inserted into the gallery space between parallel individual silicate layers separated by 2-3 nm. Conversely, the exfoliated structure results when the individual silicate layers are no longer close enough to interact with each other. In the exfoliated cases, the interlayer distances can be on the order of the radius of gyration of the polymer; therefore, the silicate layers may be considered to be well-dispersed in the organic polymer. The silicate layers in an exfoliated structure are typically not as well-ordered as those in an intercalated structure, although in many cases the exfoliated structures still bear signatures of the silicates' previous parallel registry.
Recent advances in polymer/clay and polymer/silicate nanocomposite materials have also inspired researchers to investigate polyolefin nanocomposites. Polyolefins, including polyethylene (PE), polypropylene (PP), syndiotactic polystyrene (s-PS), ethylene/propylene copolymer (EP), etc., largely prepared by Ziegler-Natta and metallocene catalysts, are a most important family of commercial polymers and have a unique combination of properties. These properties include low cost, good processability, recyclability, and a broad range of mechanical properties. Despite many commercial applications, polyolefins suffer a major deficiency, namely, poor interaction with other materials due to the lack of a polar functional group, which significantly limits their end uses, particularly those in which adhesion and compatibility with other materials is paramount (Chung “Functionalization of Polyolefins”, Academic Press, London, 2002).
As expected, dispersing the highly hydrophilic silicate clay in hydrophobic polyolefins has been a great scientific challenge to date. Many prior art processes have focused on dispersing montmorillonite-based clay in PP, a thermoplastic with an attractive combination of properties and low cost (Kato et al. J. Appl. Polym. Sci. 1997, 66, 1781; Hasegawa et al. J. Appl. Polym. Sci. 1998, 67, 87; Oya et al. J. Mater. Sci. 2000, 35, 1045; Reichert et al. Macromol. Mater. Eng. 2000, 275, 8; Maiti et al. Macromolecules 2000, 35, 2042; Svoboda et al. J. Appl. Polym. Sci. 2002, 85, 1562). The general approach of improving the incompatible polyolefin blending problem has involved the use of both functional polyolefin containing polar groups and organophilic clay (pretreated with organic surfactant). Unfortunately, the availability of functional polyolefin is very limited due to the chemical difficulties in the functionalization of polyolefins. Most of the studies were based on the commercially available maleic anhydride grafted PP (PP-g-MAH) polymers that have very complicated molecular structure due to the many impurities and the many side reactions, including severe chain degradation, that occur during the free radical grafting process. (Ruggeri et al. Eur. Polymer J. 1983, 19, 863; Hinen et al. Macromolecules, 1996, 29, 1151; Chung et al. Macromolecles 1998, 31, 5943 and 1999, 32, 2525). Typically, the formed polyolefin/clay nanocomposites have a mixed nanomorphology, with both intercalated and exfoliated structures existing in the system. A similar strategy was also applied to prepare clay/rubber nanocomposites using the combination of organophilic clay and main chain and side chain functionalized polydiene rubbers (Usuki et al. U.S. Pat. No. 5,973,053).
Overall, there were no experimental results in the prior art demonstrating the advantage of using chain end functionalized polyolefin that can be directly mixed with neat silicate clay (without pretreatment with organic surfactants) to form exfoliated polyolefin/clay nanocomposites, or to maintain this disordered clay structure even after further mixing with neat polyolefin that is compatible with the backbone of the chain end functionalized polyolefin.
In general, the chemistry for preparing a functional group terminated polymer is very limited. Usually, this type of polymer structure is prepared by a combination of living polymerization and selective termination of the living polymers with suitable reagents. This is very rare in transition metal coordination polymerization. Only a few examples of living Ziegler-Natta and metallocene mediated olefin polymerization have been reported, and these have been accomplished under very inconvenient reaction conditions using specific catalysts. (Doi et al, Macromolecules 1979, 12, 814 and 1986, 19, 2896; Yasuda et al. Macromolecules 1992, 25, 5115; and Brookhart et al. Macromolecules 1995, 28, 5378). Due to the nature of living polymerization, each initiator only produces one polymer chain, therefore the overall polymer yield is extremely low compared with that of regular Ziegler-Natta and metallocene polymerization.
Another method reported for the preparation of functional group terminated polyolefin is based on chemical modification of chain end unsaturated polypropylene (PP), which can be prepared by metallocene polymerization or thermal degradation of high molecular weight PP. (See Chung et al. Polymer 1997, 38, 1495; Mulhaupt et al. Polymers for Advanced Technologies 1993, 4, 439; and Shiono et al. Macromolecules 1992, 25, 3356 and 1997, 30, 5997). The effectiveness of this chain end functionalization process is strongly dependent on (a) the percentage of polymer chains having a vinylidene terminal group and (b) the efficiency of the functionalization reaction. It has been observed that the efficiency of the functionalization reaction decreases with an increase in PP molecular weight, due to the decrease of vinylidene concentration. Some functionalization reactions are very effective for low molecular weight PP. However, they become very ineffective for PP polymer having a molecular weight in excess of about 10,000 g/mole. Unfortunately, for many applications (e.g., one that involves improving the interfacial interactions in PP blends and composites) a high molecular weight PP chain is essential. In addition, the availability of chain-end unsaturated polyolefins is very limited and most polyolefins, except polypropylene, have a low percentage of chain end unsaturation in their polymer chains.
Another approach for preparing functional group terminated polyolefin is via in situ chain transfer reaction to a co-initiator during Ziegler-Natta polymerization. Several Al-alkyl co-initiators (Kioka et al. U.S. Pat. No. 5,939,495) and Zn-alkyl co-initiators (Shiono et al. Makromol. Chem. 1992, 193, 2751 and Makromol. Chem. Phys. 1994, 195, 3303) were found to engage chain transfer reactions to obtain Al and Zn-terminated polyolefins, respectively. The Al and Zn-terminated polyolefins can be further modified to prepare polyolefins having other terminal functional groups. However, the products comprise a complex mixture of polymers containing various end groups, due to ill-defined catalyst systems that also involve other chain transfer reactions such as β-hydride elimination and chain transfer to monomer.
In recent years, Chung has discovered a facile and general method to prepare well-defined chain end functionalized polyolefins (PE, PP, s-PS, EP, etc.) containing a terminal functional group (OH, NH2, COOH, anhydride, etc.), well-controlled polymer molecular weight, and narrow molecular weight and composition distributions (Chung Prog. Polym. Sci. 2002, 27, 39). The chemistry is centered at an in situ chain transfer reaction during metallocene-mediated α-olefin polymerization using two reactive chain transfer (CT) agents, including dialkylborane (R2B—H) (Chung, U.S. Pat. No. 6,248,837; J. Am. Chem. Soc. 1999, 121, 6764; Macromolecules 2000, 32, 8689) and styrenic molecule/H2 (Chung, U.S. Pat. No. 6,479,600; J. Am. Chem. Soc. 2001, 123, 4871; Macromolecules 2002, 35, 1622; Macromolecules 2002, 35, 9352), to form polyolefin containing a reactive alkylborane and styrenic terminal group, respectively. With an appropriate metallocene catalyst, the polymerization shows high catalyst activity, and the polymer formed shows narrow molecular weight distribution (MW/Mn˜2). The polymer molecular weight is inversely proportional to the molar ratio of [CT agent]/[α-olefin]. Furthermore, both reactive terminal groups can be quantitatively transformed to various desirable functional (polar) groups under mild reaction conditions, or even during the sample work-up step right after polymerization. The availability of a broad range of well-defined chain end functionalized polyolefins provides an advantage in evaluating their applications in polyolefin/clay nanocomposites.